The present invention relates to a parallel manufacturing system, for use in a manufacturing plant, that determines when all manufacturing operations which produce the products and components made in the plant should be performed.
In a manufacturing plant in which one or more products are made, such as an automobile assembly plant, it is necessary to schedule manufacturing operations based upon the quantities of the products that are needed and to ensure that all components used in each manufacturing operation are in inventory when that operation is scheduled to begin. Any failure to properly schedule manufacturing operations or to have the necessary components in inventory may result in late manufacture of the product and missed customer deadlines as a result.
The task of scheduling operations and ensuring a sufficient inventory of components for those operations is a complex one, particularly for a large manufacturing plant where thousands of manufacturing operations and thousands of components are used in the manufacture of numerous products. Systems that are used to determine when manufacturing operations should be performed and when components used in those operations should be in inventory are referred to herein as "manufacturing systems."
Conventional manufacturing systems in widespread use utilize a generally standard approach in addressing the above task. Manufacturing systems incorporating this standard approach are referred to herein as "MRP II" systems, and the MRP II approach is described in detail in a book entitled "Material Requirements Planning" by Joseph Orlicky and published by McGraw. Hill.
The general manner in which an MRP II system operates with respect to the manufacture of a product is as follows. The initial step is to determine the "net requirement" of the product by netting customer orders or production forecasts for the product against the current inventory of the product. The net requirements of the product is then used to determine the quantities of the components that are used in the manufacture of the product.
Components typically have different relationships to end product, depending upon when such components are used in the manufacturing process and in what operations they are used. These relationships are typically illustrated with a product tree. The product is at the highest level of the tree; all components used in the final assembly of the product occupy the next lower level in the tree; the subcomponents that are necessary to make each of those components occupy the next lower level in the tree; and so on.
After the net requirements for the product are determined, the net requirements for the components in the next lower level in the product tree are determined. This determination is based on netting the inventory of a particular component against the quantity needed as reflected by the net requirements for the product. There may also be customer orders for the component which must be taken into account. This may occur when a customer orders components used in the repair of the product.
The step of determining the net requirements for the components is again repeated for the next lower level of the product tree. The process continues until the net requirements of all components in the product tree have been determined.
After a conventional MRP II system determines the net requirements for all components needed for the manufacture of a product, the production of the required quantities of components is scheduled based upon a predetermined "lead time" required for the manufacture of each component. The lead time generally corresponds to the time required to manufacture the component, but in practice is lead time is made artificially longer.
The scheduling of the manufacture of a component is based on the lead times of all subcomponents necessary for the manufacture of that component. For example, assume that 40 units of component A must be manufactured on day 10, that component A has three subcomponents A1, A2 and A3, and that each of those subcomponents has a lead time of four days. In this case, the manufacturing operation that produces component A would be scheduled for day 10, and the manufacturing operations that produce subcomponents A1-A3 would be scheduled for day 6 so that they would be completely manufactured and ready for use in the manufacturing operation that produces component A on day 10.
The starting dates for all manufacturing operations are determined based upon the predetermined lead times and results in a rough production schedule for the plant. The production schedule is rough because there are invariably scheduling conflicts, i.e. lack of capacity to perform the manufacturing operations. Plant operators then analyze the rough production schedule and reschedule manufacturing operations in an attempt to minimize the number of conflicts.
The above process of determining the net requirements and generating the rough production schedule is typically carried out by a large mainframe computer. For relatively large manufacturing plants, the mainframe computer may take 15 hours or more to generate a rough production schedule for the plant, due to the large number of components and operations that are used in the manufacture of the products. The rough production schedule is usually generated relatively infrequently, every week for example.
There are a number of fundamental disadvantages in the operation of conventional MRP II systems as described generally above. A primary disadvantage of such systems is the failure to take into account the real-time nature of the manufacturing task. A conventional MRP II system may generate a production schedule once a week. However, during that week, plant operators are continuously rescheduling operations, ordering additional inventory, and making other day-to-day decisions that affect the production schedule. The rescheduling of an operation, in addition to affecting the components made by that operation, also affects the quantity of components consumed during that operation. Thus, a change in one operation in the system may have a relatively complex effect on the production schedule. However, since no schedule changes or other actions are accounted for until the next time the production schedule is generated, the actual day-to-day status of the manufacturing plant is not known.
Another disadvantage of MRP II systems is that lead times are typically exaggerated to ensure that all components are ready when needed. As a result, components are manufactured before they are needed. The effect of exaggerated lead times is multiplied as the MRP II system traverses down the levels of the product tree. For example, if the lead time for each component in a 10-level product tree is exaggerated by two days, the components in the lowest level of the tree will be manufactured 18 days (9 component levels*2 days per level) before they are actually needed. The exaggeration of lead times not only generates undesired excess inventory, it also further divorces the rough production plan from reality.
Conventional MRP II systems also rely on and generate a tremendous amount of unnecessary paperwork that complicates the system, impedes the ability of the system to be updated, and introduces erroneous assumptions into the system, further degrading system performance. Two examples of such paperwork are release orders and allocation orders.
A release order is an order authorizing the performance of a set of manufacturing operations. Theoretically, a release order should be generated for the date the operation is scheduled to begin. However, in practice, release orders are generated in advance of the start date of the operation to allow time for paperwork handling. The disadvantage of generating premature release orders is that it when release orders are generated, the operations affected by those orders are no longer rescheduled. Once a release order has been generated, the manufacturing system considers that order to be in progress and it cannot be rescheduled. The number of orders that cannot be rescheduled increases as the length of time that release orders are generated in advance.
After a release order is generated, an allocation request is used to reserve a quantity of each component in inventory needed for manufacture of the component specified in the release order. Once that quantity of components is allocated to the order, it is no longer considered inventory that can be used for other manufacturing operations.
One problem with allocation is that, typically, hundreds of released orders are generated before all of their components are available; consequently, work on many of the orders cannot proceed due to lack of components. Components reserved for unworkable orders end up preventing other orders from being performed. As a result, this causes sizable inventories to build up and at the same time generates rampant parts shortages.
Another problem with allocation is that the actual quantity produced on an order is very often slightly less than the exact quantity ordered. Thus, when orders are completed, a reconciliation must be made to "unallocate" components. During the time required to do this, inventory balances are incorrect, resulting in flawed material requirements planning.
Conventional MRP II systems generate paperwork when orders are released that includes a printed production order, a picking list and turnaround documents to be used in reporting production. This paperwork is dispensed to work centers by dispatchers. When any change is required in the sequence of production, or if production orders are changed or cancelled, the paperwork must be shuffled. As a result, any significant production change is greatly hampered.
Due to the above factors, such as the exaggeration of lead times, the infrequent generation of the production schedule, and inaccuracies and erroneous assumptions introduced in the paperwork trail, the integrity of the production schedule of a conventional MRP II system is severely degraded.
And when the integrity of the schedule becomes compromised, and plant operators and other personnel realize that the schedule no longer reflects when manufacturing operations must take place to make products to meet customer demands, the production schedule becomes nothing more than a formality, and production scheduling routinely takes place on an emergency, seat-of-the-pants basis.